The present invention relates to a belt for a continuously variable transmission including a large number of metal elements carried on metal ring assemblies each of which is comprised of a plurality of endless metal rings laminated one on another.
When a metal belt of a continuously variable transmission is clamped in V-faces of a drive pulley and a driven pulley, a tension is generated in the metal belt. This tension is varied by a driving force and a braking force received from both the pulleys and for this reason, the tensile stress on each of the metal rings is varied periodically with traveling of the metal belt. The traveling metal belt is bent at a pulley-wound zone and drawn out in a chord portion between the pulleys and hence, a bending stress on each metal ring is also varied periodically. As a result, a total stress (a sum of the tensile stress and the bending stress) applied to each of the metal rings is varied periodically with one rotation of the metal belt being as one cycle.
A belt for a continuously variable transmission has been proposed in Japanese Patent Application Laid-open No. 57-57938, in which a compressive residual stress is applied to an outer circumferential surface of each of the metal rings of a metal ring assembly, and a tensile residual stress is applied to an inner circumferential surface of the metal ring, whereby the middle value of the amplitude of the periodically varied stress (the middle value of the stress) on each of metal rings is brought as close to 0 as possible to prolong the fatigue life.
The innermost metal ring of such metal ring assembly abuts against a saddle surface of the metal element, while the inner circumferential surfaces of the metal rings other than the innermost metal ring abut against the outer circumferential surfaces of the other metal rings, respectively, and hence, friction coefficients of abutment portions of the innermost one and the other metal rings are of different values. Specifically, it has been ascertained by the actual measurement that a friction coefficient of the inner circumferential surface of the innermost metal ring abutting against the saddle surface of the metal element is larger than a friction coefficient of the inner circumferential surface of the metal ring other than the innermost metal ring. As a result, as will be described in detail in connection with an embodiment hereinafter, the amount of variation in tension of the innermost metal ring (a difference between the maximum and minimum values of a tension in one cycle) is larger than the amount of variation in tension of the other metal rings.
In the above conventional belt, however, the compressive residual stress is applied to the outer circumferential surfaces of all the metal rings and the tensile residual stress is applied to the inner circumferential surfaces of all the metal rings, without discrimination between the innermost metal ring and the other metal rings. Therefore, there is a problem that the durability of the entire metal belt is limited by the durability of the innermost metal ring used under a severe condition with a large amount of variation in tension. The point of start of the actual breakage is on the inner circumferential surface of the innermost metal ring and for this reason, an attention must be paid to the amplitude "sgr"a and middle value cm of the stress on the inner circumferential surface of the metal ring rather than the outer circumferential surface. The middle value "sgr"m of the stress is positive (tension) and hence, there is a problem that if the tensile residual stress is applied to the inner circumferential surface of the metal ring, the middle value "sgr"m of the stress is increased, resulting in a reduced life.
The present invention has been accomplished with the above circumstances in view, and it is an object of the present invention to prolong the fatigue life of the inner circumferential surface of the innermost metal ring which is most liable to be broken, thereby enhancing the durability of the entire metal ring assembly.
To achieve the above object, according to the present invention, there is provided a belt for a continuously variable transmission, comprising a large number of metal elements carried on metal ring assemblies each of which is comprised of a plurality of endless metal rings laminated one on another, characterized in that a compressive residual stress is applied to at least an inner circumferential surface of an innermost one of the metal rings which is in contact with a saddle surface of the metal element, the compressive residual stress applied to the innermost metal ring being larger than a compressive residual stress applied to the metal rings other than the innermost metal ring.
In addition to the above arrangement, there is provided a belt for a continuously variable transmission, wherein the compressive residual stress is applied to at least the inner circumferential surface of the innermost metal ring, so that a fatigue life defined by an amplitude "sgr"a1 and a middle value "sgr"m1 of a stress on the inner circumferential surface of the innermost metal ring is equal to a fatigue life defined by an amplitude "sgr"an and a middle vale "sgr"mn of a stress on the inner circumferential surfaces of the metal rings other than the innermost metal ring.
The function and effect according to the present invention are as follows.
The friction coefficient of the inner circumferential surface of the innermost metal ring which is in contact with the saddle surface of the metal element is larger than the friction coefficient between the metal rings which are in contact with one another and hence, the amount of variation in tension on the innermost metal ring is larger than the amount of variation in tension on the other metal rings, and the amplitude of the stress on the innermost metal ring attendant on the variation in tension is larger than the amplitude of the stress on the other metal rings. As a result, when the total stress resulting from the addition of the bending stress on the metal ring to the tensile stress on the metal ring is considered, the amplitude of the total stress on the inner circumferential surface of the innermost metal ring is larger than the amplitude of the total stress on the inner circumferential surfaces of the other metal rings, and the middle value of the stress on the inner circumferential surface of the innermost metal ring is larger than the middle value of stress on the inner circumferential surfaces of the other metal rings.
Therefore, the compressive residual stress is applied to the inner circumferential surface of the innermost metal ring used under the severest condition, and the middle value of the amplitude of the stress on the innermost metal ring is brought close to or equal to 0 (zero). Thus, the difference between the maximum value of the tensile stress and the maximum value of the compressive stress applied to the inner circumferential surface of the innermost metal ring can be decreased to enhance the durability of the innermost metal ring, thereby prolonging the life of the entire metal ring assembly.
Particularly, by setting the value of the compressive residual stress such that the fatigue life of the innermost metal ring is equal to the fatigue life of the other metal rings, the life of the entire metal ring assembly can be prolonged most effectively.